March 2017
Methodology of calculating
core inflation measures
published by Narodowy
Bank Polski
[Wpisz podtytuł dokumentu]
Economic Institute NBP
Warsaw, 2017
Published by:
Bureau of Inflation Analysis
Economic Institute NBP
Narodowy Bank Polski
00-919 Warszawa, Poland
ul. Świętokrzyska 11/21
www.nbp.pl
Introduction
Introduction
Core inflation indices serve to assess the mid- and long-term trend in the overall price level.
Despite differences in the definitions of core inflation that can be found in the literature, it is
usually assumed that core inflation is the part of registered inflation, which is related to inflation
expectations and demand pressure, and independent of supply shocks. From the point of view of
the central bank, the most important features of core inflation seem to be the following:
 It approximates the mid- and long-term trend in the growth of the prices of consumer goods
and services in the economy.
 It illustrates the trends in price changes adjusted for periodical and seasonal fluctuations, as
well as shifts resulting from supply shocks, which are usually temporary in nature.
 It points to the component of inflation that is more strongly linked with the pursued
monetary policy than is the case with the remaining components and which is consequently
sensitive to monetary policy decisions.
 Its changes enable the (ex post) assessment of the direction and scale of the impact of the
pursued monetary policy on inflation.
 Its indices are usually characterised by a smaller volatility over time than CPI inflation
indices.
The above-mentioned features of core inflation make it a useful monetary policy tool.
There are many measures of core inflation. At least three groups of core inflation measurement
methods can be distinguished: mechanical methods, based on excluding certain groups from the
basket of goods, statistical methods and methods based on formal theoretical models. At the
moment, Narodowy Bank Polski calculates and publishes four measures of core inflation. Three
of those measures are calculated using mechanical methods and one with the use of statistical
methods.
The core inflation indices allow an in-depth assessment of ongoing inflationary processes in the
economy. However, these indices are not free of shortcomings and it is difficult to indicate which
of the core inflation measures best reflects the medium-term trend in price growth. In various
economic conditions, particular measures of core inflation may approximate the non-observable
trend of the overall price level with a varying degree of accuracy. The applicability of these
measures depends, to a large extent, on the character of shocks affecting the prices of the
categories excluded from the particular measures of core inflation.
Core inflation indicators are used at Narodowy Bank Polski – similarly as in other central banks –
in its analytical, forecasting and research activities and also in the monetary policy making
process. In the Monetary Policy Guidelines, which are adopted each year, the Monetary Policy
Council asserts that “an important role in the assessment of inflationary pressure is […] played
by core inflation indices” and that “very useful in analysing supply shocks are core inflation
Methodology of calculating core inflation measures published by Narodowy Bank Polski
3
Introduction
indices, which help distinguish, at least roughly, temporary effects from permanent changes in
inflationary pressure.”
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Narodowy Bank Polski
Methodology of calculating core inflation measures
Methodology of calculating core inflation
measures
What follows is a synthetic description of how the inflation measures calculated and published
by Narodowy Bank Polski are constructed. All the indicators are calculated both in monthly
terms (in relation to the previous month), in quarterly terms (in relation to the previous quarter)
and in annual terms (in relation to the corresponding period of the preceding year). The calculations are based on a disaggregated database of consumer goods and services indicators1 as calculated by the Central Statistical Office (GUS)2. The published measures are not subject to seasonal
adjustments or smoothing procedures.
The history of publication of core inflation measures by Narodowy Bank Polski dates back to
1998. In this period, both the contents and the scope of the published measures have undergone
changes. Since March 2009 Narodowy Bank Polski has calculated and published the following
core inflation measures3:
 inflation net of administered prices,
 inflation net of most volatile prices,
 inflation net of food and energy prices,
 15-percent trimmed mean.
Measures calculated by means of mechanical methods
The most common method of calculating core inflation is the method of excluding some elementary price indicators from the overall price index of consumer goods and services (CPI). In order
to calculate this measure of core inflation, the aggregated price index has to be calculated on the
set stripped of a selected group of goods and services, which are thus excluded from the calculated measure of core inflation. Most commonly excluded goods and services are those whose prices display strong disruptions caused by supply shocks or those whose price shifts have only a
limited informative value (e.g. due to different forms of regulation). The drawback of this group
of methods is, to a certain extent, the arbitrariness of selection of the excluded group of goods.
This is the lowest available level of aggregation of price indices.
In 2017 the database published by the GUS contains 339 time series.
3 For more information on the changes introduced in core inflation measures see Revision of core inflation measures in 2009
published on the website of the NBP in section Statistics → Core inflation.
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2
Methodology of calculating core inflation measures published by Narodowy Bank Polski
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Methodology of calculating core inflation measures
This method of measure construction is used in the case of the following core inflation indicators:
Inflation net of administered prices 4
This indicator is created by eliminating prices which are not shaped by market mechanisms but
are subject to various regulations and thus the distribution of their shifts may not reflect the longterm inflation trends. In the classification of administered prices the definition used by the European Central Bank was adopted5. Administered prices include final prices of goods and services
which are administered exclusively or to a great extent by government institutions, regulators
and self-governments. The definition includes prices which result directly from legal acts or administrative decisions, which are subject to approval or may not be changed without approval of
competent government entities (e.g. regulatory authorities) and prices whose level or rate of
change are subject to legal restrictions. From this definition are excluded those prices which are
administered indirectly through changes in indirect tax rates, taxes and ecological fees, regulations in the safety of products and the Common Agricultural Policy.
A separate group of administered prices, excluded from the CPI, includes6: housing rental charges, water supply, refuse and sewerage collection services, certain energy carriers (electricity, natural gas, heat energy), passenger transport by railway and combined passenger transport7, postal
services, telephone and telefax equipment and services, other services in respect of personal
transport equipment and other (not elsewhere classified) services8. In 2017 the share (weight) of
administered prices in the CPI basket amounts to 14,7%. The inflation measure net of administered prices is constructed through calculation of the aggregated price index on the set stripped
of the group of administered prices determined with the use of the above method.
Inflation net of most volatile prices
This core inflation measure was introduced in March 20099. This indicator is constructed by
excluding the impact of most disrupted prices, i.e. prices of those goods and services which are
subject to various demand and supply shocks and/or which are characterised by strong seasonal
In the core inflation indicator net of regulated prices, published until the end of 2008, the category of regulated prices, different from the currently used category of administered prices, was used. Regulated prices included: (1) prices administered
directly or indirectly by government institutions, local governments or regulators, (2) services provided by government monopolies: transport, postal and telecommunications services, insurance, and (3) prices largely impacted by excise tax (fuels,
alcohol, tobacco).
5 For the definition of administered prices, see Annex no. 2 and information in the document published on the website of the
ECB: http://ec.europa.eu/eurostat/web/hicp/methodology/hicp-administered-prices
6 Scope of the administered prices in the subsequent years shows the Table 2 in Annex 2
7 Combined transport includes periodical fares for bus, tramway and underground transport.
8 The group of other services includes such services as: legal services, services of government authorities, funeral and cemetery
services.
9 Inflation measure net of the most volatile prices replaced previously used two inflation measures (one used since 1998 - the
inflation measure net of the most volatile prices and second used since 2000 – the inflation measure net of most volatile prices
and fuel prices) which were calculated using different methodology.
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Narodowy Bank Polski
Methodology of calculating core inflation measures
shifts, variable over time. As a result of strong volatility of certain prices, the CPI, at least within a
short-time period, may not accurately indicate the long-term growth trend of the overall price
level. For this reason, the core inflation measure net of most volatile prices is constructed.
The basket of the most volatile prices is determined by the use of the empirical method, and this
basket is adjusted, at the beginning of every year, for the data released in the preceding year. The
criterion of standard deviation of the sample at the level of elementary price indicators was used
to determine the basket of most volatile prices10. Every year the basket of the most volatile prices
includes goods and services, starting from the most volatile ones until their weight reaches 20%
of the CPI basket. The inflation measure net of the most volatile prices is constructed through
calculation of the aggregated price index on the set stripped of the basket of the most volatile
prices determined with the use of the above method. Thus determined group of the most volatile
prices includes mainly11: a significant part of food products (mainly unprocessed and lowprocessed fruit, vegetables and meat), energy products (fuels, gas, heating fuels), Internet access
services, public administration services and legal services.
Inflation net of food and energy prices12
This indicator of core inflation eliminates the prices of goods that are particularly sensitive to
internal and external supply shocks. This group includes prices of food and non-alcoholic beverages and prices of energy. The group of energy prices includes: engine fuels and energy carriers
(electricity, natural gas, central heating, hot water, heating fuels)13. In 2017 the share (weight) of
food and energy prices in the CPI basket amounts to 40.4%14. The inflation measure net of food
and energy prices is constructed through calculation of the aggregated price index on the set
stripped of the basket of food and energy prices.
Measures calculated by means of statistical methods
15-percent trimmed mean
The only measure calculated with the use of statistical methods is the 15-percent trimmed mean.
It is a weighted average, calculated according the methodology used for calculating the CPI on
the set of price groups whose accumulated weights (corresponding to previously sorted in an
Standard deviation is calculated using the time series of annual growth of elementary groups’ prices over the past three years.
Then, elementary groups are arranged in a descending order according to this criterion.
11 As a result of using the method of determining most volatile prices this basket is variable in time.
10
12
In connection with the publication of the series of core inflation net of food and energy prices, starting from June 2008, the
NBP after having published the data for December 2008 ceased to publish core inflation net of food and fuel prices.
13 According to the international classification COICOP used by the GUS to calculate the Consumer Price Index, the group
“Fuels and lubricants for personal transport equipment” (“engine fuels”) is included in the group “Transport” (COICOP 07),
while the group “Electricity, gas and other fuels” (“energy carriers”) belongs to the group “Housing, water, electricity, gas and
other fuels” (COICOP 04).
14 See: Table 1 in Annex 1.
Methodology of calculating core inflation measures published by Narodowy Bank Polski
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Methodology of calculating core inflation measures
ascending order price indicators) are higher than 15% and lower than 85%. The distribution of
price changes is trimmed symmetrically on both sides. The groups of prices that have changed
most significantly in relation to the base period, with the total weight of 30%, are eliminated in
this way. In contrast to the other measures published by the NBP, the 15-percent trimmed mean
is calculated independently on a monthly and annual basis, i.e. it cuts off those groups of prices
which were characterised by the largest changes both in relation to the previous month and in
relation to the corresponding month of the preceding year.
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Narodowy Bank Polski
Annex 1 - share (weights) in the CPI of groups of goods and services excluded from particular core inflation measures
Annex 1 - share (weights) in the CPI of groups
of goods and services excluded from particular
core inflation measures
Table 1 Shares (percentage weights) in the CPI of groups of goods and services excluded from
particular core inflation measures (2001-2017)
Year
Administered
prices
Most volatile
prices
Food and nonalcoholic
beverages
Energy
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
18,1
19,5
20,2
20,6
20,8
18,2
12,7
12,0
12,0
12,4
12,7
13,9
14,3
14,9
14,9
14,9
14,7
20,1
21,5
20,2
20,5
20,4
20,6
20,4
20,6
20,3
20,5
20,4
21,5
21,6
20,2
20,0
20,3
20,3
30,1
29,7
28,2
27,0
26,9
27,2
26,2
25,7
24,6
24,1
24,0
24,2
24,3
24,6
24,4
24,0
24,2
13,9
14,5
14,6
14,9
14,4
15,3
16,1
15,0
15,3
15,8
16,9
17,6
17,8
18,2
17,5
17,1
16,1
Source: NBP calculations based on GUS data
Methodology of calculating core inflation measures published by Narodowy Bank Polski
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Annex 2 - Administered prices
Annex 2 - Administered prices
The ECB’s definition of administered prices
According to the ECB’s definition, administered prices cover those components of consumer
goods and services whose final prices are fully or mainly influenced by government (central, regional and local government) and regulators. Administered prices include:
 Fully administered prices cover the prices of goods and services that are directly set by government (e.g. public transport charges, fees for administrative documents).
 Mainly administered prices cover the prices of goods and services which government or another national regulator have significant influence on. These may include, for example, prices that can not be changed without prior approval of a national regulator or public authority.
What is important, influence of such decisions may have a direct bearing on retail prices or
indirect via wholesale prices. However, the regulator must have a very significant impact on
the consumer price.
Covered in the definition of administered prices are:
 Price changes approved by governments and national regulators.
 The effects of restrictions in the consumer price level (price caps/ price floors), if binding.
 The effects of permanent (long-term) restrictions on consumer price changes (price growth
regulation).
 Social rents offered at a heavily subsidised price level to, for example, low-income households.
 Fixed, Government-set fees for health, education and other social services in so far as covered in the HICP basket.
Not covered in the ECB’s definition of administered prices are:
 Consumer prices subject to indirect taxation and excise duties (goods and services with a
high indirect tax component in the final price e.g. tobacco products, fuel).
 Effects of product regulations such as safety or environmental standards.
 Prices subject to Common Agricultural Policy of the European Union.
 Index-linked prices, unless the reference indicator is an administered price or when the index-linking is mandatory and enforced by regulation and law.
 The effects of transitory restrictions on consumer price changes (e.g. restrictions in place in
some countries at the time of the changeover to the euro).
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Narodowy Bank Polski
Annex 2 - Administered prices
For telecommunications services, the ECB adopted the following definition:
 Fixed line and internet services: if price regulation exists, and is binding, at the retail level,
(for at least 50% of the consumption expenditures), fixed line services should be considered
as mainly administered. If price regulation exists only at the wholesale level only, fixed line
and internet services should be classified as non-administered prices unless there are arguments (quantitative or qualitative) for its dominant impact on retail prices.
 Mobile telephone services are considered as non-administered prices unless there are arguments (quantitative or qualitative) for a dominant impact of price regulation at the wholesale
level on retail prices, or if the share of revenues from regulated rates is above 50% of total
revenues of mobile operators.
For gas and electricity prices, the ECB adopted the following definition:
 The HICP sub-indices for electricity and gas cover both a service component (transmission
and distribution of electricity and gas) and commodity component (electricity and gas itself).
 In the case where only the service component, and not the commodity component, falls under the above definition of an administered price, the whole subindex shall not be considered as an administered price.
 In the case where both the service and the commodity component fall under the above definition of an administered price, the whole subindex should be considered as an administered price.
Table 2 Administered subcomponents included in the CPI index in Poland
COICOP
Description
Time period
411
Actual rentals paid by tenants
throughout the whole period
412
Other actual rentals
throughout the whole period
441
Water supply
throughout the whole period
442
Refuse collection
from 2012
443
Sewerage collection
throughout the whole period
451
Electricity
throughout the whole period
452
Gas
throughout the whole period
455
Heat energy
throughout the whole period
724
Other services in respect of personal transport
equipment
from 2015
731
Passenger transport by railway
throughout the whole period
735
Combined passenger transport
throughout the whole period
810
Postal services
throughout the whole period
820
Telephone and telefax equipment
until 2006
830
Telephone and telefax services
until 2006
1270
Other services, not elsewhere classified
throughout the whole period
Note: green frames concern groups which at present are not considered to be administered prices. Source: NBP
Methodology of calculating core inflation measures published by Narodowy Bank Polski
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